Ceramic material and method of manufacture

ABSTRACT

A dual phase silicon aluminum oxynitride material comprising a first phase Si-Al-O-N, commonly referred to as β--Si--Al--O--N, and a second phase Si-Al-O-N referred to as α--Si--Al--O--N. In addition to the double phase Si-Al-O-Ns, there is included a glassy type material which can formulate up to ten percent by weight of the total composition. The material may be manufactured by forming a polytype material made from reacted alumina, aluminum nitride and silicon nitride. The polytype material may be mixed with further powders of silicon nitride and an oxide of yttrium, lithium or calcium and finally reacted to a double phase Si-Al-O-N material where hardness is increased by the additional α-Si-Al-O-N is increased significantly affecting its strength. 
     The material may be formed in situ by mixing aluminum nitride, alumina, silicon nitride, together with an oxide of yttrium, lithium or calcium. These materials can then be sintered to a final product containing a double phase Si-Al-O-N. Control of the alumina content in the polytype or in situ methods affects the percentage of α-Si-Al-O-N produced in the final product. The hardness of the material increases with the α-Si-Al-O-N content without significantly affecting its transverse rupture strength.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 867,910,filed May 23, 1986, now abandoned, which is a continuation ofapplication Ser. No. 800,955, filed Nov. 22, 1985, now abandoned, whichwas a division of application Ser. No. 466,716, filed Feb. 15, 1983, andnow U.S. Pat. No. 4,563,433, which was a continuation-in-part ofapplication Ser. No. 351,289, filed Feb. 22, 1982, now abandoned.

BACKGROUND OF THE INVENTION

This invention concerns Si-Al-O-N type materials and is moreparticularly concerned with the double phase Si-Al-O-N useful for themanufacture of cutting inserts used in metalworking.

There are numerous papers and patents describing the relatively newSi-Al-O-N material which have been created by the addition of thealuminum and oxygen atoms to silicon nitride materials.

Most recently, these materials have found their way into themetalworking industry and have provided possibilities in the working ofcast iron, nickel based super alloys, and other similar substances.

More particularly, cutting inserts of a Si-Al-O-N type material made inaccordance with U.S. Pat. No. 4,127,416, which is incorporated herein byreference, have proven to be useful in certain metal-working situations.The type of material made by the above-mentioned United States patent ismanufactured as a predominantly single phase β-Si-Al-O-N material withapproximately 10 to 20 percent of a glassy phase present.

The material is made essentially as described in the patent whichinvolves forming a polytype material as an initial step in the process.The polytype material may then be reacted with a controlled amount ofsilicon nitride and an oxide of yttrium, lithium or calcium to form aceramic of at least 80, and preferably 95, percent being of a singlepase β-type Si-Al-O-N.

Such a material, when produced, has a transverse rupture strength in therange of 100,000 to 110,000 pounds per square inch using the proceduredescribed in later examples and a knoop hardness in the range of 1450 to1800 kilograms per square millimeter at 100 grams load.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, a double phase Si-Al-O-N material isproduced, especially for use as a cutting insert material. The doublephases are comprised of α and β phase Si-Al-O-Ns.

It was discovered that control of the amount of alumina in the mixturetogether with a neutral media for milling allows one to control thecomposition of the final material such that the α and β type Si-Al-O-Nphases will appear. Less alumina produces a greater amount of α phaseSi-Al-O-N. Control of the other starting components will also producethe same effect, such as less silica, more aluminum nitride, morepolytype, increased yttria all produce more α Si-Al-O-N in the finishedproduct. Preferably, the α phase Si-Al-O-N will range from 10 to 70percent by weight, while the β phase Si-Al-O-N in the composition willrange from 20 to 90 percent by weight. A glassy phase ranging from zeroto 10 percent by weight will also be present.

Increasing the α-Si-Al-O-N in the composition causes the hardness to beincreased without significantly affecting the transverse rupturestrength of the material.

Compounds of yttrium are used as sintering aids in the manufacture ofthe above-mentioned product, but it is to be recognized that similarresults could be obtained with oxides of scandium, cerium, lanthanum andthe elements of the lanthanide series.

Use of the yttria as the preferred sintering aid gives rise to anintergranular component predominantly comprising a glass phase but whichmay also comprise other phases which include YAG (yttriuma luminumgarnet) which is a cubic phase having the formula Y₃ Al₅ O₁₂ ;Y-N-α-Wollastonite, which is a monoclinic phase of formula YSiO₂ N; YAM,which is a monoclinic phase of the formula Y₄ Al₂ O₉ ; N-YAM, which is amonoclinic phase of formula Y₄ Si₂ O₇ N₂ which is isostructural with YAMand forms a complete solid solution with it.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of the invention will become more clearly apparent uponreference to the following detailed specification taken in connectionwith the accompanying drawings in which:

FIG. 1 shows the silicon nitride corner of the base plane of theSi-Al-O-N phase diagram as defined in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention concerns a dual phase Si-Al-O-N ceramic and glassy phaseproduct and method of making said product which comprises the steps offorming a powder mixture consisting essentially of a first componentconsisting of compounds containing the elements silicon, aluminum,oxygen and nitrogen in proportions such that the ratio of the totalnumber of silicon and aluminum atoms to the total number of oxygen andnitrogen atoms lies in the range 0.735 to 0.77 and such that saidcompounds react, together with the second component, during thesubsequent sintering process to produce a double phase ceramic materialwith a first phase obeying the general formula Si_(6-z) Al_(z) O_(z)N_(6-z) where z is between 0.38 and 1.5, and a second phase being anhexagonal phase and obeying the general formula (Si, Al)₁₂ M_(x) (O,N)₁₆ where M can be calcium, or yttrium, or any of the lanthanides, andx is between 0.1 and 2. The second component comprises between 0.1 and10 percent based on the total weight of the first and second components,the second component being an oxide of at least one of the furtherelements yttrium, scandium, cerium, lanthanum, and the metals of thelanthanide series. Said mixture is then sintered in a protective(protective meaning non-reactive) environment with or without theapplication of pressure at a temperature between 1600 degrees Centigradeand 2000 degrees Centigrade and for a time, decreasing with increasingtemperature, of at least ten minutes to at least five hours so as toproduce a ceramic material containing at least 90 percent by volume ofsaid double phase ceramic material with said second phase containingpart of said second component.

In the method described in the preceding paragraph, the compounds of thefirst component are arranged so that the sum of all the silicon andaluminum atoms in the compounds divided by the sum of all the oxygen andnitrogen atoms present is between 0.735 and 0.77, or more preferably0.745 to 0.76. The two component mixture is then sintered in aprotective (protecting meaning non-reactive) environment, preferably anon-oxidizing environment, or more preferably, a reducing environment,at 1600 degrees Centigrade to 2000 degrees Centigrade for a timesufficient to produce at least 90 percent by volume of the siliconaluminum oxynitride ceramic material defined by the above formulae. Thesintering time required increases with decreasing temperature so that,although the minimum time is only ten minutes in the case of a 2000degrees Centigrade sintering temperature, with a temperature of 1600degrees Centigrade, a sintering time of at least five hours is required.

The components forming the first component of the original mixture areconveniently silicon nitride, aluminum nitride, alumina and silica, withat least part of the silica and alumina being present as inherentimpurities on the silicon nitride and aluminum nitride, respectively.

Alternatively, the first component may be defined by silicon nitride anda ceramic intermediary containing a silicon aluminum oxynitride whichdoes not obey the general formula Si_(6-z) Al_(z) O_(z) N_(6-z). Suchmaterials are referred to as polytypes and are described and defined inU.S. Pat. No. 4,127,416, which has already been incorporated herein byreference. Examples 7 through 17 utilize the 21R type polytype definedin said U.S. patent. Preferably, the silicon aluminum oxynitride of theceramic intermediary has a rhombohedral structure and obeys theapproximate formula SiAl₆ O₂ N₆. Moreover, the ceramic intermediary ispreferably formed by heating a powder mixture of alumina, aluminum andsilicon to between 1200 degrees Centigrade and 1400 degrees Centigradein a nitriding atmosphere, the heating rate being controlled tosubstantially prevent exotherming, and then sintering the nitridedmixture at a temperature between 1500 degrees Centigrade and 1900degrees Centigrade.

Alternatively, the intermediary may be formed by heating a powdermixture of alumina, aluminum nitride and silicon nitride at atemperature between 1200 degrees Centigrade and 2000 degrees Centigradein a protective environment, preferably a nonoxidizing environment, ormore preferably, a reducing environment.

In the methods described above, the relative proportions of thecompounds present in the mixture are arranged so as to produce the dualphase ceramic material with a first phase obeying the formula Si_(6-z)Al_(z) O_(z) N_(6-z) and a second phase obeying the formula (Si, Al)₁₂M_(x) (O, N)₁₆ where z is between 0.38 and 1.5 since having the z valuewithin these limits is found to produce a coherent product having a highstrength even when the sintering is performed in the absence ofpressure. If, on the other hand, the z value is allowed to fall below0.38, the material becomes difficult to sinter without the applicationof pressure, while the strength of the product deteriorates if the zvalue is allowed to increase above 1.5.

Moreover, the relative proportions of the compounds in the firstcomponent are arranged so as to provide the above defined atomic ratioof between 0.735 and 0.77 since, if the ratio falls below 0.735, it isfound that the mixture becomes too oxygen-rich. This results in theproduction of an excessive amount of glass during sintering which notonly has a deleterious effect on the high temperature strengthproperties of the product, but is also found to adversely affect the lowtemperature strength properties. Moreover, it is found that the glasscannot be removed by the subsequent heat treatment process discussed indetail below. By way of contrast, if said atomic ratio exceeds 0.77, itis found that there is insufficient oxygen present to form the glassrequired to effect consolidation of the product.

The permitted range of 0.1 to 10 percent by weight for the secondcomponent of the starting mixture is also chosen on the basis that itprovides a satisfactory glass content in the sintered product. Theelements selected for the second component are cerium, yttrium,scandium, lanthanum or one of the lanthanide series since these havehighly refractory oxides which produce high melting point glasses withthe silica and alumina present and hence allow the product to be used athigher temperature than would be possible with low melting pointglasses. The second component is also necessary for the formation of theα-Si-Al-O-N phase of the first component since, by definition, theα-Si-Al-O-N contains yttrium or one of the lanthanides. Of the elementsselected for the second component, yttrium is preferred, since thepresence of yttria in the sintering mixture is found to result inproducts of high strength even without the application of pressure.

It will be seen that performing the methods described above results inthe formation of a sintered ceramic product containing at least 90percent by volume of a dual phase silicon aluminum oxynitride, togetherwith an intergranular component predominantly comprising a glassy phasebut also possibly containing other phases such as YAG, YAM, N-YAM andY-N-α-Wollastonite. The presence of glass aids consolidation of theproduct during sintering, but tends to result in a lowering of the hightemperature properties of the final component. It has, however, beenfound that the amount of the glass phase in the sintered product can bereduced by subjecting the product to a final heat treatment processwhich involves raising the temperature of the product to within 200degrees Centigrade of the melting point of the glass (i.e., to about1400 degrees Centigrade in the case of an yttrium glass), and thencooling the product to crystallize at least part of the glass into anintergranular component containing other phases such as YAG, YAM, N-YAMand Y-N-α-Wollastonite.

EXAMPLES

Starting materials used in this application are as listed below, but canbe the same starting materials listed in the aforementioned Lucas U.S.Pat. No. 4,127,416, or any of the other known starting materials meetingthe known conditions of the manufacture of Si-Al-O-N materials.

Silicon (Elkem Metals)

Fe<1.0%

C 0.1-0.4% typical

Ca<0.07% typical

Al<0.53%

-200 mesh particle size

Yttrium (Molycorp, a division of Union 76)

99.99% pure

-325 mesh particle size

Aluminum (Alcan Aluminum Corporation)

99.3% pure

16 micron average particle size

Alumina (Reynolds)

RC-172DBM

99.7% Al₂ O₃

0.04% Na₂ O

0.07% SiO₂

0.03% Fe₂ O₃

particle size<1 micron

Alumina (Alcoa)

A-16SG

99.5% Al₂ O₃

0.05-0.09% Na₂ O

0.02-0.04% SiO₂

0.01-0.02% Fe₂ O₃

particle size<1 micron

In Table 2, the percent α-Si-Al-O-N and β-Si-Al-O-N was originally basedon 100 percent, since no other crystalline phases were present andignored the 10 percent glass which cannot be quantified by x-raydiffraction. The percentages were revised to include the 10 percentglass and, therefore, the percentages α-Si-Al-O-N and β-Si-Al-O-N willtotal 90 percent, making the percentages consistent with Table 2(continued). Depending upon the convention one chooses, the percentageswill be correct.

EXAMPLE 1

A composition consisting of 92 parts by weight silicon nitride powder(containing about 4 weight percent surface silica), 5 parts by weight ofaluminum nitride (containing about 6 weight percent surface alumina), 5parts by weight of alumina and 7 parts by weight of yttrium oxide wasmilled in isopropanol for 96 hours using Si-Al-O-N media to a meanparticle size of 0.96 microns. Following drying, the powder was screenedthrough a 50 mesh sieve and isostatically pressed at 30,000 psi. Piecesof green material were cut from the isostatically pressed slug andburied in a 50/50 by weight boron nitride and silicon nitride powdermixture inside a graphite pot. The pot was placed in a graphite elementresistance-heated furnace and raised to 500 degrees Centigrade in vacuumand then to 1830 degrees Centigrade in one atmosphere pressure ofnitrogen at which temperature it was held for 40 minutes. After coolingbars of the sintered material 0.2×0.2×0.8 inches were ground using a 600grit abrasive wheel and, following die checking, they were broken in 3point bend with an outer span of 0.56 inches. Broken pieces were usedfor density and hardness measurements and phase determination by x-raydiffraction. Properties of the material are given in Table 1.

EXAMPLE 2

As Example 1, but sintered at 1830 degrees Centigrade for 60 minutes.

EXAMPLE 3

A composition consisting of 92 parts by weight silicon nitride powder(containing about 4 weight percent surface silica) 5 parts by weight ofaluminum nitride (containing about 6 weight percent surface alumina) 3parts by weight of alumina and 7 parts by weight of yttrium oxide wasmilled in isopropanol using alumina grinding media for 48 hours.Attrition from such media amounted to 1.9 parts by weight, whichincorporated into the overall composition. The mean particle size of themilled powder was 1.49 microns. The powder was processed as in Example1, except that sintering was carried out at 1780 degrees Centigrade for40 minutes and 1830 degrees Centigrade for 15 minutes. Properties aregiven in Table 1.

EXAMPLE 4

A composition consisting of 92 parts by weight silicon nitride powder(containing about 4 weight percent surface silica), 8 parts by weight ofaluminum nitride (containing about 6 weight percent surface alumina),and 7 parts by weight of yttrium oxide was milled in isopropanol for 168hours using dense Si-Al-O-N media to a mean particle size of 0.63microns. Then as in Example 1.

EXAMPLE 5

Material, as in Example 4, was given a heat-treatment of 1400 degreesCentigrade for five hours in a static nitrogen atmosphere. Results inTable 1.

EXAMPLE 6

A composition consisting of 92 parts by weight of silicon nitride powder(containing about 4 weight percent surface silica), 8 parts by weight ofaluminum nitride (containing about 6 weight percent surface alumina),and 5 parts by weight of yttrium oxide was milled in isopropanol usingalumina grinding media for 48 hours. Attrition from such media amountedto 2.0 parts by weight which was incorporated into the overallcomposition. The mean particle size of the milled powder was 1.47microns. Then as in Example 1, but sintered at 1850 degrees Centigradefor 60 minutes. Results in Table 1.

POLYTYPE EXAMPLES EXAMPLE 7

A powder mixture was made up comprising 86.9 w/o (weight percent)silicon nitride, 6.59 w/o 21R polytype and 6.54 w/o yttria. The powdermixture was then milled for two days utilizing Si-Al-O-N cycloids as themedia until the resulting average particle diameter was 1.07 microns and90 percent finer than 2.21 microns. The powder was then coldisostatically pressed at 30,000 psi, and the green slug was thensintered under the same conditions as the previous examples at 1830degrees Centigrade for 50 minutes.

The sintered material was then analyzed and properties are given inTable 2.

EXAMPLE 8

The powder was processed as described in Example 7 except that thestarting powder mixture consisted of 81.3 w/o silicon nitride, 12.1 w/o21R polytype, 6.54 w/o yttria. Sintered material was analyzed andproperties are given in Table 2.

EXAMPLE 9

The processing of this powder was the same as in Examples 7 and 8,except that the ball milling media was alumina cycloids. The originalpowder mixture was 86.9 w/o silicon nitride, 6.54 w/o 21R polytype, and6.54 w/o yttria. The powder was milled at an average particle diameterof 0.91 microns and 90 percent finer than 1.72 microns. It was foundthat the powder mixture had an additional 3.55 w/o milled pick-up fromthe alumina cycloids.

The mix was then sintered at 1780 degrees Centigrade for forty minutesand 1830 degrees Centigrade for 25 minutes. The sintered material wasanalyzed and properties are given in Table 2.

EXAMPLE 10

The powder mixture was processed with 82.2 w/o silicon nitride, 11.2 w/o21R poltype, 6.54 w/o yttria and an additional 3.57 w/o from wear ofalumina cycloids during ball milling. The average particle diameter was0.93 microns with 90 percent finer than 1.77 microns after milling. Thiscomposition was sintered on the same schedule as Example 9. The sinteredmaterial was then analyzed and properties are given in Table 2.

EXAMPLE 11

The powder mixture was processed with 85 w/o silicon nitride, 8.4 w/o21R polytype, 6.54 w/o yttria, and a direct addition of 2.51 w/o aluminaand 0.1 w/o of silica. The mix was milled with Si-Al-O-N media to anaverage diameter of 1.0 microns. The sintered material was then analyzedand properties are given in Table 2.

EXAMPLE 12

A composition consisting of 83 parts by weight Si₃ N₄ (with 1.0 w/o O asa surface layer), 17 parts by weight 21R polytype, 7 parts by weightyttria and 3 parts by weight alumina was milled in isopropanol for 72hours using Si-Al-O-N media to a mean particle size of 0.71 microns.Following drying, the powder was screened through a 50 mesh sieve andisostatically pressed at 30,000 psi. Pieces of green material were cutfrom the isostatically pressed slug and buried in a 75/25 by weightsilicon nitride and boron nitride powder mixture inside a graphite pot.The pot was placed in a graphite element resistance heated furnace andraised to 900 degrees Centigrade under vacuum and then to 1780 degreesCentigrade for 40 minutes in one atmosphere nitrogen followed by 25minutes at 1830 degrees Centigrade and cooled in approximately 30minutes to 1000 degrees Centigrade. Properties are given in Table 2.

EXAMPLE 13

A composition consisting of 77 parts by weight silicon nitride (with1.09 w/o O as a surface layer) 23 parts by weight 21R polytype, 7 partsby weight yttria and 3 parts by weight alumina. Processing was identicalto Example 12. The mean particle size of the milled powder was 0.84microns. Properties are given in Table 2.

EXAMPLE 14

A composition consisting of 75 parts by weight silicon nitride (with1.09 w/o O as a surface layer), 25 parts by weight 21R polytype, 7 partsby weight yttria and 3 parts by weight alumina. Processing was identicalto Example 12. The mean particle size of the milled powder was 0.92microns. Properties are given in Table 2.

EXAMPLE 15

A composition consisting of 75 parts by weight silicon nitride (with0.77 w/o O as a surface layer), 25 parts by weight 21R polytype, 7 partsby weight yttria and 9 parts by weight aluminum oxide. Processing wasidentical to Example 12. The mean particle size of the milled power was0.82 microns. Properties are given in Table 2.

EXAMPLE 16

A composition consisting of 85 parts by weight silicon nitride, 15 partsby weight 21R polytype, 7 parts by weight yttria and 1.0 parts by weightalumina. Processing was identical to Example 12. The mean particle sizeof the milled powder was 0.95 microns. Properties are given in Table 2.

EXAMPLE 17

A composition consisting of 85 parts by weight silicon nitride, 15 partsby weight polytype, 7 parts by weight yttria and 8 parts by weightalumina. Processing was identical to Example 12. The mean particle sizeof the milled powder was 1.09 microns. Properties are given in Table 2.

The composite material produced in the above examples showed superiormetalcutting results when used as a cutting insert. Superior resultswere obtained when machining cast iron and nickel base alloys. Testresults reported in the tables for the first eleven examples reportedthe transverse rupture strength of the material as determined by themethod described in the Lucas Industries U.S. Pat. No. 4,127,416 and thedimensions specified in Example 1 of the specification.

Subsequently, it was decided that fracture toughness of the material wasa much better indication of metal-cutting ability for the material thanthe transverse rupture values. For new Examples 12 through 17, thesevalues are now reported instead of the transverse rupture values.

The fracture toughness tests used a Vickers diamond indentation with an18 kilogram load. Fracture toughness was calculated from the dimensionsof the indentation and associated cracks together with the load and aYoungs modulus value of 305 G Pa using the method described in A. G.Evans and E. A. Charles Journal of the American Ceramic Society, Volume59 (1976), Page 371.

Examples 10, 12, 13 and 14 demonstrate the increase in percentα-Si-Al-O-N with increasing polytype. Examples 16 and 17, 14 and 15demonstrate the decrease in α-Si-Al-O-N content and hardness withalumina content.

The present invention is further defined with reference to FIG. 1.Reference is had to Lucas Industries U.S. Pat. Nos. 4,127,416 and4,113,503, in which the Si-Al-O-N phase diagram is shown.

The rectangular composition area claimed by Lucas is outlined in theattached drawing. The boundaries are set at z values of 0.38 and 1.5,where "z" is found in the formula for β-Si-Al-O-N of Si_(6-z) Al_(z)O_(z) N_(8-z). The upper and lower boundaries are cation to anion ratios(c/a) of 0.735 and 0.770. Lucas defined the c/a ratio as moles siliconplus moles aluminum divided by the quantity moles oxygen plus molesnitrogen. The contribution of yttria was not included. Exceeding theupper c/a ratio, results in too much glass, which is deleterious toproperties of the single phase β-Si-Al-O-N. Sintering the single phaseβ-Si-Al-O-N is difficult with ratios higher than 0.770.

The compositional area, which overlaps the Lucas area, was defined withdistinct differences. The boundaries set at z=0.38 and z=1.5 are commonwith Lucas, but the upper and lower boundaries are based on the presenceof a two-phase ceramic, α-Si-Al-O-N plus β-Si-Al-O-N. The c/a ratio isdefined as moles silicon plus moles aluminum plus moles yttrium dividedby the quantity moles oxygen plus moles nitrogen. Yttria is included inthe c/a ratio, which is appropriate since yttrium is an integral part ofα-Si-Al-O-N.

Second, the equivalents calculated by Lucas considers only Si, Al, O, N,excluding Y₂ O₃. The present compositions have equivalent calculatedwith yttria, which result in a compositional point slightly above thebase plane on the phase diagram. The compositional point is thenprojected onto the base plane resulting in an effective equivalent forsilicon and aluminum. Oxygen and nitrogen will not be affected. Theeffective equivalents are plotted in FIG. 1. The table below shows thedifferences between lucas and the present method for Example 9.

                  TABLE I                                                         ______________________________________                                               Si    Al      O       N     Y     c/a                                  ______________________________________                                        Lucas    .9299   .0701   .0553 .9447 --    .747                               Equivalent                                                                    Equivalent                                                                             .9102   .0687   .0753 .9247 .0211 .744                               (including                                                                    Y.sub.2 O.sub.3)                                                              Effective                                                                              .9208   .0793   .0753 .9247 --    --                                 Equivalent                                                                    ______________________________________                                    

In this manner, the compositional region is defined on the base plane,but is indirectly accounting for the influence of yttria, which isimportant since yttria enters the α-Si-Al-O-N structure.

The upper boundary segment with a constant c/a ratio of 0.739 representsthe effective equivalent compositions of a final composition between0-10% α-Si-Al-O-N. Examples 17 and 15 define a line O_(eff) eq=0.1644(Al_(eff) eq)+0.0865, which intersects the line of constant c/aof 0.739 at (0.1143, 0.1053) and the line of z=1.5 at (0.2084, 0.1208).The combination of the c/a ratio 0.739 line segment with the segmentbetween the points of intersection described above represents thecompositions with an effective equivalent percent that result in a finalα-Si-Al-O-N content of 0-10%. The lower boundary represents a constantc/a ratio of 0.794. The ratio corresponds to the compositional range forα-Si-Al-O-N with the maximum practical yttrium substitution in theα-Si-Al-O-N structure. The general α-Si-Al-O-N formula, proposed by K.H. Jack, in "The Role of Additives in the Densification of NitrogenCeramics," (October 1979), for European Research Office, United StatesArmy Grant No. DAERO-78-G-012, is Y_(x) Si₁₂₋(m+n) Al_(m+n) O_(n)N_(16-n) where x=.sup. 0-2, m=1-4 and n=0-2.5.

DEFINITION OF PHASES

1. β' is an hexagonal phase having the general formula Si_(6-z) Al_(z)O_(z) N_(8-z) where 0<z<4.2. Detected by X-raydiffraction-characteristic patterns for z=0 and z=4β'.

2. α' is an hexagonal phase having the general formula (Si, Al)₁₂ M_(x)(O, N)₁₆ where M=Li, Ca, Y or other lanthanides. Theoretical maximum isx=2; this is approached in the case of Ca but, for Y, practical maximumis about 0.7. Detected by X-ray diffraction.

3. α-Si₃ N₄ is an unsubstituted allotrope of Si₃ N₄.

4. N-YAM is an monoclinic phase of formula Y₄ Si₂ O₇ N₂. Isostructuralwith YAM-Y₄ Al₂ O₉ and forms a complete solid solution with it.

5. Y-N-α-Wollastonite is a monoclinic phase of formula YSiO₂ N.

6. YAG is a cubic phase of formula Y₃ Al₅ O₁₂. Some substitution of Alby Si and simultaneous replacement of O by N may occur.

                                      TABLE 1                                     __________________________________________________________________________    Transverse                                                                    Rupture    Rockwell "A"                                                                          Knoop Hardness                                             Strength   Hardness (at                                                                          (100 g load)                                                                           Density                                                                            Phases Present                               Example                                                                            (psi) 60 kg load)                                                                           (kg mm.sup.-2)                                                                         (g cm.sup.-3)                                                                      % β'                                                                        % α-Si.sub.3 N.sub.4 /α'                                                 Other                              __________________________________________________________________________    1    106,000                                                                             93.2    1940     3.266                                                                              81  9     Y-containing glassy phase,                                                    trace                                                                         N--YAM (Y.sub.4 Si.sub.2                                                      O.sub.7 N.sub.2)                   2    115,000                                                                             93.4    1890     3.271                                                                              84  6     Y-containing glassy phase,                                                    trace                                                                         N--YAM (Y.sub.4 Si.sub.2                                                      O.sub.7 N.sub.2)                   3     87,000                                                                             92.5    1730     3.203                                                                              80 10     Y-containing glassy phase,                                                    N--YAM (Y.sub.4 Si.sub.2                                                      O.sub.7 N.sub.2) and               Wollastonite (YSiO.sub.2 N)                Y--N-- α                     4    100,000                                                                             94.6    2150     3.275                                                                              47 43     Y-containing glassy phase,                                                    N--YAM (Y.sub.4 Si.sub.2                                                      O.sub.7 N.sub.2) and               Wollastonite (YSiO.sub.2 N)                Y--N-- α                     5     96,000                                                                             94.8    2310     3.280                                             Wollastonite, trace              49 41     N--YAM, Y--N-- α                                                        YAG (Y.sub.3 Al.sub.5 O.sub.12)                                               9                                  6    --    93.0    1980     3.175                                                                              58 32     Y-containing glassy phase,                                                    N--YAM                             __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________               Rockwell "A"                                                                          Knoop Hardness                                                        Hardness (at                                                                          (100 g load)                                                                           Density                                                                            Phases Present                               Example    60 kg load)                                                                           (kg mm.sup.-2)                                                                         (g cm.sup.-3)                                                                      % β'                                                                        % α-Si.sub.3 N.sub.4 /α'                                                 Other                              __________________________________________________________________________         Transverse                                                                    Rupture                                                                       Strength                                                                      (psi)                                                                     7    83,485                                                                             92.9    --       3.25 74.7                                                                             15.3   Y-containing glassy phase                                                     with                                                                          no intergranular crystalline                                                  phases                              8   106,785                                                                             94.7    1761      3.276                                                                             34.2                                                                             55.7   Y-containing glassy phase                                                     with                                                                          no intergranular crystalline                                                  phases                              9   111,990                                                                             92.9    1718     3.27 76.0                                                                             14.0   Y-containing glassy phase                                                     with                                                                          no intergranular crystalline                                                  phases                             10    94,856                                                                             94.0    --       3.26 54.0                                                                             36.0   Y-containing glassy phase                                                     with                                                                          no intergranular crystalline                                                  phases                             11   111,596                                                                             93.3    1765     3.25 63.9                                                                             26.1   Y-containing glassy phase                                                     with                                                                          no intergranular crystalline                                                  phases                                  Fracture                                                                      Toughness                                                                12   7.59  94.0    1632     3.28 51.3                                                                             38.7   No other phases present            13   7.23  93.9    1611     3.30 44.3                                                                             45.7   No other phases present            14   7.32  94.2    1598     3.30 44.5                                                                             45.5   No other phases present            15   7.44  93.0    1546     3.30 80.4                                                                              9.6   No other phases present            16   6.90  94.5    1680     3.27 31.5                                                                             58.5   No other phases present            17   5.72  92.9    1503     3.26 90 --     No other phases                    __________________________________________________________________________                                               present                        

Modifications may be made within the scope of the appended claims.

What is claimed is:
 1. A metalcutting insert made by the method ofproducing a ceramic product, comprising the steps of forming a powdermixture consisting essentially of a first component consisting ofcompounds containing the elements of silicon, aluminum, oxygen andnitrogen in proportions such that the ratio of the total number ofsilicon and aluminum atoms to the total number of oxygen and nitrogenatoms lies in the range 0.735 to 0.77 and such that said compounds reacttogether with the second component during the subsequent sinteringprocess to produce a double phase ceramic material wherein a first phaseobeys the general formula: Si_(6-z) Al_(z) O_(z) N_(8-z) where z isbetween 0.38 and 1.5; wherein a second phase obeys the general formula(Si, Al)₁₂ M_(x) (O, N)₁₆ ; and wherein up to 10 percent by weight of asecond component is in the form of an oxide of at least one of thefurther elements, yttrium, scandium, cerium, lanthanum and the metals ofthe lanthanide series; and wherein sintering said mixture in anon-reactive environment comprises placing said mixture in a graphitepot and covering with a mixture of boron nitride and silicon nitridepowders while surrounding said graphite pot and said silicon nitride andboron nitride powders with a nitrogen gas, with or without theapplication of pressure, at a temperature between 1600 degreesCentigrade and 2000 degrees Centigrade and for a time, decreasing withincreasing temperature, of at least ten minutes to at least five hoursso as to produce a ceramic material containing said double phase ceramicmaterial together with a second phase containing said at least onefurther element, which is a glassy phase.
 2. A metalcutting insert whichis comprised of an α and β phase composite of the Si-Al-O-N material anda glassy phase wherein said α phase Si-Al-O-N ranges from 10 to 70weight percent of said ceramic material, said β phase Si-Al-O-N rangesfrom 20 to 90 weight percent of said ceramic material and said glassyphase ranges from 0.1 to 10 weight percent of said ceramic material. 3.A metalcutting insert of claim 2 which further consists of said ceramicmaterial having a minimum hardness of 92.5 Rockwell "A" as measured witha 60 kilogram load.
 4. A metalcutting insert according to claims 2 or 3wherein said glassy phase may also contain a crystalline phase from thegroup consisting of YAG, YAM, N-YAM and Y-N-α-Wollastonite.
 5. Ametalcutting insert which consists essentially of an α and β phasecomposite of the Si-Al-O-N material and a glassy phase wherein said αphase Si-Al-O-N ranges from 10 to 70 weight percent of said ceramicmaterial, said β phase Si-Al-O-N ranges from 20 to 90 weight percent ofsaid ceramic material and said glassy phase ranges from 0.1 to 10 weightpercent of said ceramic material.
 6. A metalcutting insert of claim 5which further consists of said ceramic material having a minimumhardness of 92.5 Rockwell "A" as measured with a 60 kilogram load.
 7. Ametalcutting insert according to claims 5 or 6 wherein said glassy phasemay also contain a crystalline phase from the group consisting of YAG,YAM, N-YAM and Y-N-α-Wollastonite.